A recently discovered human herpesvirus, HHV-8 (also called Kaposi sarcoma associated herpesvirus, KSHV), has been implicated as a causative factor in Kaposi sarcoma and certain B cell lymphomas. Permissive systems for replication of HHV-8 have not been identified. While HHV-8 is related to herpesvirus saimiri and has been placed in the same rhadinovirus subgrouping, an animal model that uses HHV-8 or a closely related virus has not been defined. A herpesvirus that is related to but distinct from HHV-8 was isolated from rhesus monkeys. The sequence of 10.6 kbp from virion DNA revealed the presence of an interlukin-6 homolog similar to what is present in KSHV and a closer relatedness of the DNA polymerase and glycoprotein B reading frames to those of KSHV than to those of any other herpesvirus. This rhesus monkey herpesvirus replicated lytically and to high titers in cultured rhesus monkey fibroblasts. Antibody testing revealed a high prevalence for at least 10 years in our rhesus monkey colony and a high prevalence in two other colonies that were tested. Thus, rhesus monkeys naturally harbor a virus related to KSHV, which we have called RRV, for rhesus monkey rhadinovirus.